The fouling of membrane surfaces during membrane filtration often causes a decline in the production of filtered water. Accordingly, various means are traditionally employed to keep membrane surfaces clean and to reduce fouling rates. Most known membrane cleaning techniques employ the principle of increasing cross-flow, i.e., flow in a tangential direction, of the liquid to be filtered relative to the membrane surface to be cleaned. The increased cross-flow causes turbulent flow and increases the shear stresses across the membrane surface, thereby dislodging solids accumulated on the surface.
The efficacy of cleaning using cross-flow filtration may be further improved if the cross-flow consists of a continuous or cyclic two-phase mixture of gas and liquid, with the gas dispersed in the liquid phase in the form of gas bubbles ranging in size from about 0.1 mm to 10 mm. For example, the application of continuous two-phase gas-liquid mixtures is illustrated in the submerged membrane system described in U.S. Pat. No. 5,639,373. The two-phase flow causes additional turbulence and mechanically agitates the membrane surface, resulting in increased shear stress across the membrane surface due to increased relative velocity between the membrane surface and the liquid to be filtered. As a result, the efficacy of cleaning the membrane surface increases.
Alternatively, the cyclic two-phase gas-water flow described in U.S. Pat. No. 6,245,239 increases the cleaning efficiency of the membranes by allowing the liquid to be filtered to cyclically accelerate or decelerate, thus avoiding dead zones in the mass of liquid, which are common in continuous two-phase gas-water flow. Membranes can also be cleaned by the application of injected pressure pulses and low frequency ultrasound (Cheryan M.: Ultrafiltration and Microfiltration Handbook (1998) and Water Treatment Membrane Processes, Joel Mallevialle, Ed. (1996)). All of these cleaning procedures are advantageous in that they may be performed without the need for interrupting the membrane filtration process.
In contrast, membrane cleaning using backwashing requires that the filtration process be interrupted. Backwashing involves pumping a stream of permeate (filtered liquid) through the membrane wall in a direction opposite to the flow direction during filtration, thereby dislodging the particles deposited on the membrane surface. Air may also be used instead of permeate for backwashing, as described for example in U.S. Pat. No. 6,159,373.
These known methods for cleaning and/or maintaining clean membrane surfaces have several disadvantages. For example, one disadvantage of increasing turbulence and shear stress across a membrane surface by increasing the relative velocity of the filtered liquid with respect to the membrane surface is that significant amounts of energy are required. Significant power consumption is also involved in generating the ultrasonic field strength required to carry out ultrasonic cleaning of a membrane surface. Accordingly, such methods may not be applicable in cases where high relative velocities cannot be applied between the membrane surface and the liquid to be filtered.
Additionally, cleaning methods using two-phase continuous or cyclic gas/liquid flow typically utilize significant quantities of air for aeration and therefore such processes may not be applicable in cases where the liquid to be filtered needs to be maintained under anaerobic conditions. Further, the small size of the gas bubbles and their non-linear movement may cause undesirable foam-formation. The small gas bubbles may get trapped in the matrix of solid particles present in the feed stream, float, and separate from the bulk of the feed stream, resulting in a non-homogeneous feed solution.
Finally, cleaning a membrane surface by permeate or gas backwashing necessitates interruption of the membrane filtration-process, thus resulting in downtime and loss of productivity. The backwashing process is also suitable only for certain membrane types and membrane configurations. For example, plate and frame membranes cannot be backwashed since the backwash pressure may peel the membrane off of the support frame.
Accordingly, there remains a need in the art for an improved, economical method of cleaning and maintaining membrane surfaces which may be performed during filtration and which would be as effective as known cleaning methods, yet applicable to a variety of different membrane configurations, including those maintained under anaerobic conditions.